1. Field of the Invention
The present invention relates to a sheet processing apparatus, such as an image forming apparatus provided with a LBP (laser beam printer) or a copying machine, which is capable of recording images on sheets, such as paper sheets, using the electrophotographic printing method, and a bookbinding device as an application device of the copying machine, such as a finisher, a sorter, or a stacker, a method of controlling the sheet processing apparatus, a control program for implementing the control method, and a storage medium storing the control program.
2. Description of the Related Art
In conventional bookbinding devices, the number of sheets that can be stapled e.g. by a sorter or a finisher which is capable of performing stapling processing varies depending on the sheet size and the sheet thickness.
In the case of a so-called plain sheet having a predetermined thickness e.g. of 0.1 mm, the maximum number of A4-size sheets that can be stapled as a bundle is 50, whereas that of A3-size sheets that can be stapled as a bundle is only 40.
In the case of a so-called thick paper sheet being thicker than a plain sheet and having a thickness e.g. of 0.2 mm, the maximum number of A4-size sheets that can be stapled as a bundle is 30, whereas that of A3-size sheets that can be stapled as a bundle is only 20.
Thus, the number of sheets that can be bundled varies depending on the sheet type, and the maximum processable number of sheets is determined based on the size and thickness of the sheet.
Information on sheet thicknesses is entered by the user or registered in advance as fixed values in the apparatus. Some bookbinding apparatuses measure the thickness of each sheet when the sheet is fed.
Japanese Laid-Open Patent Publications (Kokai) Nos. H07-165363 and H08-301504 disclose a mechanism that detects the thickness of a bundle of sheets. According to the mechanism disclosed in the former publication, the thickness of a sheet bundle in one processing section is detected, and when the thickness of the sheet bundle reaches a predetermined value, subsequent sheets are received and aligned in another processing section, and discharged out of an apparatus including the mechanism without being bound. According to the mechanism disclosed in the latter publication, based on a result of detection by detection means for detecting the thickness of a sheet bundle stored in a tray, sheets to be guided into a sheet tray are temporarily stored at a predetermined position.
Further, Japanese Laid-Open Patent Publications (Kokai) Nos. H10-007314 and No. 2000-318918 disclose an apparatus that detects the thickness of a bundle of sheets and controls the position of a guide for sandwiching the bundle and carrying out processing thereafter, and an apparatus that performs stapling processing by selectively using a plurality of stapling devices, respectively.
Furthermore, there has been-proposed a tape-binding device e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H10-181236, in which several types of binding tapes different in width are selectively supplied to a tape heating device according to the thickness of a sandwiched sheet bundle.
In the above described prior art in which the upper limit of the number of sheets to be bundled or that of the thickness of the sheet bundle is set according to the sheet type (sheet thickness). However, in the case of the upper limits being specified based on weight, the limit of sheet thickness cannot be determined accurately due to difference between the measure of weight and that of sheet thickness, and hence limit values have to be set with an allowance for the upper limit of the number of sheets to be bundled or that of sheet bundle thickness.
For this reason, to enable a predetermined number of e.g. A4-size plain sheets of any type to be bundled insofar as they are categorized as A4-size plain sheets, the number of A4-size plain sheets that can be bundled is limited to 50 even when up to 100 of them can be bundled in actuality. Thus, even plain sheets of a size that can actually be processed as a bundle of up to 100 sheets are categorized into a group having an upper limit of 50 sheets for bundling.
Similarly, even an apparatus which is capable of detecting sheet thickness on a sheet-by-sheet basis, stores not information on the thickness of each sheet, but only information on categories of sheet types, such as plain sheet, thick paper, and so forth. Therefore, the upper limits of the number or thickness of sheets of types in a category, which can be processed as a bundle, are determined to be within an upper limit thereof set for the category.
In recent years, the type of sheets for use in an image forming apparatus has become diversified, which causes diversity in sheet thickness within each category. In response to this tendency, measures are being taken e.g. to increase the number of categories.
However, even if the number of types to be set is sufficiently increased, bundling is executed based on preset upper limits of the number of sheets and sheet bundle thickness. Therefore, assuming that sheets smaller in thickness than an assumed sheet thickness are to be bundled, even after sheets are stored in a predetermined number corresponding to the upper limit for bundling, there still remains room for increasing the number of sheets that can be bundled. On the other hand, assuming that sheets larger in thickness than an assumed sheet thickness are to be bundled, when sheets are stored for bundling in a number corresponding to the upper limit, the thickness of a bundle of the sheets exceeds the maximum allowable thickness, which can cause degradation of finishing due to occurrence of wrinkling, faulty binding, or the like.
As far as detection of sheet thickness is concerned, a sheet is heated and pressed for fixing while passing through a fixing device, and therefore the thickness of a sheet is smaller when the sheet has passed through the fixing device than that detected before passing through the fixing device. This is because heat from the fixing device evaporates water contained in the sheet, and pressure applied to the sheet compresses or flattens spaces within the sheet. Further, since a sheet bundle is pressed and sandwiched for bundling, and the sheet bundle is further pressed for bonding spaces between a plurality of sheets to be bundled are eliminated, whereby the thickness of the sheet bundle changes.
That is, in conventional bundling, it is general that sheet thickness detected before fixing is used in bundling which should be carried out after measurement of the thickness of a sheet bundle.
Therefore, in view of finishing of bookbinding, it is preferred that the thickness of a sheet bundle is measured after fixing and during bundling.